Download Sex Linkage and Recombination

Sex Linkage and Recombination
by CHED on June 16, 2017
lesson duration of 10 minutes
under General Biology 2
generated on June 16, 2017 at 04:33 pm
Tags: Sex Linkage and Recombination
CHED.GOV.PH
K-12 Teacher's Resource Community
Generated: Jun 17,2017 12:33 AM
Sex Linkage and Recombination
( 1 hour and 10 mins )
Written By: CHED on July 17, 2016
Subjects: General Biology 2
Tags: Sex Linkage and Recombination
Resources
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Content Standard
1. Mendel’s Laws of Inheritance
2. Sex Linkage
3. Central Dogma of Molecular Biology
4. Recombinant DNA
Performance Standard
1. make a pedigree analysis in the learner’s family using a simple genetic trait
2. make a research paper/case study/poster on genetic diseases
3. make a diagram (e.g., pictogram, poster) showing the evolution of a domesticated crop
4. differentiate the 3-Domain Scheme from the 5-Kingdom Scheme of classification of living things
Learning Competencies
Explain sex linkage and recombination
Introduction 5 mins
Communicating Learning Objectives
1. Cite the learning objectives, which are as follows:
I. illustrate the transmission of an X-linked and a Y-linked character
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II. compute the probability of the occurrence of a sex-linked trait
III. give examples of other sex-related traits
Relevant Vocabulary
2. State the relevant vocabulary:
I. Sex linked trait. The gene (pair) that determines a character (e.g. hemophilia) is located on the sex chromosomes
II. X-linked trait. A sex-linked trait is where the gene or allele for the trait is found on the X chromosome
III.Color
Colorblindness.
blindness.An
AnX-linked
X-linkedrecessive
recessivetrait
traitwhere
whereaaaffected
affectedindividual
individualcould
couldnot
notdistinguish
distinguishred
redfrom
fromgreen
greencolor
color
III.
(red green color blindness)
Hemophilia.
X-linked
recessive
where
affected
individual
suffers
from
delayed
blood
clotting
during
IV.IV.
Hemophilia.
AnAn
X-linked
recessive
traittrait
where
an an
affected
individual
suffers
from
delayed
blood
clotting
during
injuries because of the absence of certain blood clotting factors
V. Y-linked trait. A sex-linked trait where the gene or allele for the trait is found on the Y chromosome
VI. Hypertrichosis pinnae auris. A Y-linked trait where affected males have hair growing from their external ears
VII. Other sex-related traits.
A. Sex-influenced
Sex-influenced traittrait- Any
Any trait
trait in
in aa diploid
diploid organism
organism whose
whose expression
expression is
is affected
affected by
by an
an individual’s
individual’s biological
biological sex;
sex; aa
A.
trait that occurs at a higher frequency in one sex over the other
B. Sex-limited trait- Any trait in a diploid organism whose expression is limited to just one biological sex
Teacher tip:
Ask the learners to review the topic on recombination in Meiosis that they took up in BIO 1. Recombination or
shufflingofofgenes/
genes/alleles
allelesininMeiosis
Meiosisresults
resultstotovariation
variationininthe
thegenome
genomeofofgametes,
gametes,the
thesperm
spermcells
cellsand
andegg
egg
shuffling
cells.
In cell
any of
cell
ofbody
the body
(somatic),
are chromosome
In humans,
pair numbers
are the
In any
the
(somatic),
therethere
are chromosome
pairs.pairs.
In humans,
pair numbers
1-22 1-22
are the
autosomes or body chromosomes while the last (23rd) pair is the sex chromosome.
Normal human
human females
females have
have two
two X
X chromosomes
chromosomes and
and normal
normal human
human males
males have
have one
one X
X chromosome
chromosome and
and aa Y
Y
Normal
chromosome; that is: XX- female XY- male
Motivation 10 mins
Case Study
Present these three cases using pictures:
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Use
Use aa high
high resolution
resolution figure
figure (photograph
(photograph or
or image
image projected
projected on
on aa computer
computer or
or LCD)
LCD) to
to ensure
ensure the
the accuracy
accuracy of
of the
the color
color
blindness
Those
could
the figure
normal;
those
cannot
colorblind.
In most
cases,
blindness
test.test.
Those
that that
could
see see
the figure
are are
normal;
those
that that
cannot
are are
colorblind.
In most
cases,
the the
colorblindmales
malesoutnumber
outnumberthe
thecolorblind
colorblindfemales,
females,which
whichare
arerare.
rare.If Ifthere
thereare
areno
nocolorblind
colorblindindividuals
individualsininthe
theclass,
class,
colorblind
the teacher will just have to mention as a matter of fact that colorblind females are rare.
Be careful
in conducting
to discourage
teasing
of actual
colorblind
learners.
Emphasize
colorblind
Be careful
in conducting
this this
test test
to discourage
teasing
of actual
colorblind
learners.
Emphasize
that that
colorblind
individuals are normal except that they could not distinguish between red and green colors.
Misconception: Common
Common
misconception
is that
baldness
occurs
in males.
Emphasize
baldness
does
misconception
is that
baldness
occurs
onlyonly
in males.
Emphasize
thatthat
baldness
does
happen in women, although the frequency is much lower and is therefore rare.
Instruction
25 mins
Sex-linked traits
Give the definition of an X-linked trait
Explain why X-linked traits may occur more frequently in one sex over the other
In humans, males and females are represented by different sex chromosomes
Females have two X chromosomes in the nucleus of their cells.
Males have one X chromosome and one Y chromosome in the nucleus of their cells.
Depending on whether the trait is dominant or recessive, the expression pattern of the trait differs in males
and females
Colorblindness in humans as an example of sex-linked trait
The alleles responsible for colorblindness is found on the X chromosome only
The dominant allele is the normal allele; the recessive allele causes colorblindness
Females need two copies of the recessive allele, one from each of the two X chromosomes, for the trait to be
manifested. If they only have one copy of the recessive allele, they have normal color vision. However, they
are carriers for the trait in that they may pass it on to their offspring.
Males only need one recessive allele in their sole X chromosome for the trait to be expressed.
Explain what happens to the expression patterns if the trait is X-linked and dominant.
Use Table 2 as guide.
Give the definition of a Y-linked trait
Explain why there is difference in expression between males and females for Y-linked traits. (Since the allele
is found only in the Y chromosome, and since only males have Y chromosomes, then only males will express
the trait. Females CANNOT express Y-linked traits.)
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Hypertrichosis pinnae auris as an example of a Y-linked trait
If a male has the allele responsible for the trait, then his Y chromosome will possess that allele. Since he will
pass on his Y chromosome to his sons, then all his sons will inherit the trait, and they, in turn, can pass on
the allele to their sons.
3. Describe other sex-related traits
Sex-influenced trait
Give the definition
Explain why traits may be expressed differently between sexes
Hormonal or physiological differences between the sexes cause differences of expression of certain genes
Baldness in humans as an example of a sex-influenced trait. See Table 1 how baldness is hypothesized to
be expressed by a single pair of alleles, with B as the dominant allele for baldness and b as the recessive
normal allele.
Sex-limited traits
Give the definition
Explain why traits may be limited to one sex only
Hormonal or physiological differences between sexes may limit the expression of some genes to one
biological sex only
Functional mammary glands as an example of a sexlimited trait. Only females can express functional
mammary glands that produce milk immediately after giving birth.
Note that baldness behaves like a dominant trait in males in that only one dominant allele is needed for
baldness to be expressed. On the other hand, the trait behaves like a recessive trait in women in that they
need both dominant alleles to be present for baldness to be expressed.
Practice 20 mins
1. Divide learners into groups of four.
2. Ask each group to answer a set of questions related to sex-related traits in humans. See sample questions.
Enrichment 10 mins
As a homework, provide this narrative to the class:
The last Emperor of Russia, Nicolas II, was married to Empress Alexandra, and they had five children, Olga, Tatiana,
Maria, Anastasia, and Alexis. Alexis was the only one who was afflicted with hemophilia or the royal bleeding disease;
all other members were normal
Research on this medical condition and determine the mode of inheritance.
If only Prince Alexis was afflicted with the disease, determine his genotype.
What could be the genotypes of the Emperor and Empress?
Is it possible that each daughter could have been a carrier?
Teacher tip:
Hemophilia is an X-linked recessive trait. Empress Alexandra was most likely a carrier of the trait (XCX). She
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was a descendant of Queen Victoria of the United Kingdom, who herself was a probable carrier. The Emperor
was completely unaffected and therefore had an XY genotype. Based on the genotypes of the parents, Alexis
had an XCY genotype, with the defective X chromosome carrying the allele for hemophilia coming from his
mother. Each daughter, in turn, had a 50% probability of being a carrier, but they could NEVER have been
affected
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